1. Technical Field
The invention relates generally to the continuous casting of metals. More particularly, the invention relates to the protection of reactionary metals from reacting with the atmosphere when molten or at elevated temperatures. Specifically, the invention relates to using a molten material such as liquid glass to form a barrier to prevent the atmosphere from entering the melting chamber of a continuous casting furnace and to coat a metal casting formed from such metals to protect the metal casting from the atmosphere.
2. Background Information
Hearth melting processes, Electron Beam Cold Hearth Refining (EBCHR) and Plasma Arc Cold Hearth Refining (PACHR), were originally developed to improve the quality of titanium alloys used for jet engine rotating components. Quality improvements in the field are primarily related to the removal of detrimental particles such as high density inclusions (HDI) and hard alpha particles. Recent applications for both EBCHR and PACHR are more focused on cost reduction considerations. Some ways to effect cost reduction are increasing the flexible use of various forms of input materials, creating a single-step melting process (conventional melting of titanium, for instance, requires two or three melting steps) and facilitating higher product yield.
Titanium and other metals are highly reactive and therefore must be melted in a vacuum or in an inert atmosphere. In electron beam cold hearth refining (EBCHR), a high vacuum is maintained in the furnace melting and casting chambers in order to allow the electron beam guns to operate. In plasma arc cold hearth refining (PACHR), the plasma arc torches use an inert gas such as helium or argon (typically helium) to produce plasma and therefore the atmosphere in the furnace consists primarily of a partial or positive pressure of the gas used by the plasma torches. In either case, contamination of the furnace chamber with oxygen or nitrogen, which react with molten titanium, may cause hard alpha defects in the cast titanium. Thus, oxygen and nitrogen should be completely or substantially avoided within the furnace chamber throughout the casting process.
In order to permit extraction of the casting from the furnace with minimal interruption to the casting process and no contamination of the melting chamber with oxygen and nitrogen or other gases, current furnaces utilize a withdrawal chamber. During the casting process the lengthening casting moves out of the bottom of the mold through an isolation gate valve and into the withdrawal chamber. When the desired or maximum casting length is reached it is completely withdrawn out of the mold through the gate valve and into the withdrawal chamber. Then, the gate valve is closed to isolate the withdrawal chamber from the furnace melt chamber, the withdrawal chamber is moved from under the furnace and the casting is removed.
Although functional, such furnaces have several limitations. First, the maximum casting length is limited to the length of the withdrawal chamber. In addition, casting must be stopped during the process of removing a casting from the furnace. Thus, such furnaces allow continuous melting operations but do not allow continuous casting. Furthermore, the top of the casting will normally contain shrinkage cavities (pipe) that form when the casting cools. Controlled cooling of the casting top, known as a “hot top”, can reduce these cavities, but the hot top is a time-consuming process which reduces productivity. The top portion of the casting containing shrinkage or pipe cavities is unusable material which thus leads to a yield loss. Moreover, there is an additional yield loss due to the dovetail at the bottom of the casting that attaches to the withdrawal ram.
The present invention eliminates or substantially reduces these problems with a sealing apparatus which permits continuous casting of the titanium, superalloys, refractory metals, and other reactive metals whereby the casting in the form of an ingot, bar, slab or the like can move from the interior of a continuous casting furnace to the exterior without allowing the introduction of air or other external atmosphere into the furnace chamber.